Modern electronic devices such as smartphones and high resolution displays are made possible by advances in semiconductor technology. Inside these modern electronic devices lie components known as integrated circuits (IC). ICs provide the processing power and memory that allow the electronic devices to do complex tasks quicker.
ICs are manufactured in semiconductor fabrication facilities (fabs) that employ a variety of semiconductor capital equipment tools maintained in a carefully monitored environment. The capital equipment tools perform hundreds of process steps such as implantation, deposition, photolithography, etching, polishing, and packaging to transform a semiconductor wafer into a number of integrated circuit chips.
The process steps involved in the fabrication of ICs need to be carefully monitored and controlled to maximize production yield. Yield is a percentage value that refers to the ratio of the number of ICs that meet performance specifications in a given batch to the total number of ICs produced in the batch. Maximizing production yield of ICs is important to produce cost-effective electronic devices.
A variety of wafer inspection and metrology tools are employed in semiconductor fabs for monitoring and controlling process steps. Optical tools are used for tasks such as detecting defects on wafers and photolithography masks and for measuring wafer shape, critical dimension, film properties, and overlay.
Detecting yield-limiting defects on wafers, identifying the root cause of such defects, and controlling process parameters to bring defect levels within limits, are important activities in semiconductor fabrication. Advances in semiconductor fabrication technologies have reduced the size of individual components inside ICs from micrometer scale to nanometer scale features. As a result, the size of yield-limiting defects have also come down to the nanometer scale.
Detecting nanometer scale defects with traditional dark field or bright field optical inspection techniques is challenging because nanometer scale defects barely scatter optical radiation. The scattering intensity of defect particles, being proportional to the sixth power of defect diameter, exhibits a strong dependence on defect size. For example, reducing defect size by a factor of 2 results in a reduction in scattering intensity by a factor of 64. In order to detect yield-limiting defects at leading-edge semiconductor technology node, a leading-edge defect inspection tool requires an exponential increase in defect signal detection performance compared to a previous generation tool.
Scattering intensity of defect particles, being inversely proportional to the fourth power of wavelength, exhibits a dependence on wavelength. For example, reducing the wavelength of optical radiation by a factor of 2 increases scattering intensity by a factor of 16. However, developing light sources with high brightness at wavelengths lower than deep ultra violet has been challenging.
Scattering intensity of defect particles also exhibits a linear dependence on the intensity of optical radiation incident on the defects. A factor of 2 increase in the intensity of optical radiation increases scattering intensity also by a factor of 2. However, increasing the intensity of optical radiation increases the risk of damaging the wafer. The risk of laser damage is particularly high in ultraviolet wavelengths where the absorption coefficient of Silicon is high compared to visible and infrared wavelengths. Furthermore, increasing the optical power of a laser increases its complexity, size, and cost.
Accordingly, reducing the wavelength and increasing the intensity of optical radiation are not viable alternatives to adequately compensate the exponential decrease in scattering intensity of decreasing yield-limiting defect sizes. While semiconductor technology node sizes decreased at a fast pace from 130 nm to 14 nm (over 9× reduction) in the last decade, minimum detectable defect sizes decreased at a slower pace from 50 nm to 20 nm (2.5× reduction) in the same time period. This slower pace of improvement in defect detection performance negatively affects the production yield, cost, and timely release of leading-edge semiconductor devices.
Throughput of wafer inspection refers to the number of wafers that can be inspected per hour. Achieving high throughput is desirable in semiconductor fabs to reduce production times and cost.
In traditional wafer inspection tools, there exists a tradeoff between their defect sensitivity and inspection throughput. An increased ability to detect tiny defects comes at the price of reduced throughput. Similarly, an increased inspection throughput comes at the price of reduced sensitivity to tiny defects. This unfortunate tradeoff in traditional wafer inspection tools requires one to choose between two entities that are often equally important to semiconductor fabs.
Dark field wafer inspection tools illuminate a laser spot on the surface of a wafer and collect scattered light originating from the spot. Specular reflection from the spot is carefully prevented from entering the collection optics. Semiconductor wafers are polished to be smooth, so an overwhelming majority of light incident on the spot undergoes specular reflection and is prevented from being collected. The scattered light is so small that it is measured in the units of parts per million. Only a few of the millions of incident photons are eventually collected and detected. The majority of incident optical radiation is wasted. Accordingly, dark field wafer inspection tools suffer from extremely low optical efficiencies.
Darkfield wafer inspection tools inherently suffer from low inspection throughput because of scanning. The diameter of wafers to be inspected can be as large as 450 mm, while the size of the spot illuminated on the wafer is only a few tens of micrometers in size. In order to inspect every point on the wafer, the spot needs to be sequentially scanned to as much as a billion different points of the wafer.
In traditional dark field wafer inspection, techniques to improve inspection throughput by increasing the spot size of inspection results in a decrease in defect sensitivity because of two reasons. Firstly, the increase in spot size leads to a reduction in intensity of incident optical radiation on a defect, leading to a proportional reduction in the intensity of scattered light from the defect. Secondly, increasing the spot size leads to an increase in a background nuisance signal called haze. Haze refers to scattered radiation from surface roughness in wafer. Although the magnitude of surface roughness in a wafer is smaller than yield-limiting defect sizes, the somewhat uniform presence of surface roughness throughout the spot area (as opposed to isolated defects) could create a net haze signal large enough to overwhelm the signal from defects. This in turn leads to reduced defect sensitivity.
In traditional dark field wafer inspection, techniques to improve inspection throughput by increasing the speed of scanning also results in a decrease in defect sensitivity. The higher the scanning speed, the smaller the amount of time the spot spends at each point on the wafer. The amount of scattered radiation collected and detected from a point on the wafer reduces as the amount of time the spot spends at the point is reduced. Therefore, increasing scanning speed leads to a reduction in defect signal, leading to a reduction in defect sensitivity.
Traditional dark-field illumination systems used for wafer inspection suffer from a number of disadvantages. a) reduced defect sensitivity; b) reduced inspection throughput; c) trade-off between defect sensitivity and inspection throughput; d) extremely low optical efficiency; e) large and complex laser sources and illumination systems; f) complex scanning systems; g) change in wafer properties such as shape due to high speed scanning; h) reduced reliability because of motion of components during imaging; and i) increased cost.
Accordingly, there is a need for an improved illumination system in wafer inspection systems that can improve defect sensitivity; improve inspection throughput; break the trade-off between defect sensitivity and inspection throughput; offer high optical efficiencies; reduce complexity of laser sources and illumination systems; reduce complexity of scanning systems; preserve wafer properties such as wafer shape during inspection; increase reliability; and reduce cost of the illumination system.